Widget Manage in Customizer

I am a WP newbie so my knowledge is limited yet expanding. I am trying to get a Main Alternative Sidebar to appear on one of my pages using the Manage in Customizer. However, I do not see an option on how to add it. This Main Alternative Sidebar exists on other pages so I know it works. All I see on this one page is Main Sidebar (column 1). SO how do I accomplish this? TIA.